Trolls are not just "Nelves with tusks", how utterly disingenuous. You cannot possibly look at the Night Elves and think that the only thing that sets them apart from trolls is the lack of tusks.
Trolls would add a much-needed primitive and bestial side to the Alliance.
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What are these clues? And are these clues from Vanilla, 20 years ago, back when the Old Gods would have made Sargeras "beg for the mercy of a swift death"?
Either way, even if trolls somehow predated the arrival of the Old Gods, troll civilization does not. Before King Dazar's time, the trolls had no form of civilization, while the Black Empire was indeed the first time that a race on Azeroth organized in a civilized and structured manner. We know the Black Empire predate Dazar by countless millennia as the Great Seal, built by the Titan keepers to contain G'Huun and the very foundation of the Zandalari Empire itself, predates Troll civilization as we know it.